In such a “jack pump” mechanism, a piston and non-return-valve unit at the base of the well or borehole (which may be several thousand meters deep) is generally connected to a drive mechanism at the surface by means of a long steel rod that, being assembled in sections and screwed together, is known as a string. The topmost section of that string—the section that emerges from the well through a pressure seal—necessarily has a higher surface finish and is known as the polished (or polish) rod.
It has long been known to construct such mechanisms in the form of an oscillating horizontal beam having a hammer-shaped end, over which is wrapped a chain or cable from which the pumping string is suspended. The opposite end of the beam is generally driven up and down by a cranked motor mechanism. Other, more complex, mechanisms exist in which, for example, the polished rod is suspended from a pulley or belt and in which the pulley or belt is raised and lowered by the rotation and contra-rotation of a winch mechanism. Nevertheless, the reciprocating beam or “nodding donkey” has remained the most popular device for moving a rod string. Reciprocating beams have been in use since the eighteenth century for pumping water from mines and the world's first steam engine was designed to drive such a pump.
It will also be understood that the long steel rod “string” that connects the drive mechanism at the top of the well with the pump itself at the base of the well has a deadload mass of several tonnes, which must also be supported by the beam. To increase the efficiency of the mechanism, the deadload has to be counterbalanced.
Although some recent mechanisms have used a gas spring (in the form of a pneumatic cylinder as a “prop” beneath the loaded end of the beam) it is more common for the counterbalance to be in the form of eccentric weights, attached to the shaft of the crank mechanism that drives the oscillating beam. (It will be recognised, however, that such a counterbalance technique doubles both the system inertia and the static bearing load.)
From earliest times the pumping stroke has traditionally been about ten feet (two or three meters) and the pumping frequency has been around 5 (between 1 and 10) strokes a minute. It will be understood that the traditional choices have been determined by the large masses involved and by the asymmetric action of the device, which places high stress on the parts and causes significant wear on the bearings of the “nodding donkey” mechanism.
The traditional machine has many moving parts and it is required to operate for 24 hrs a day, 365 days a year for several years and so it needs regular inspection, lubrication, maintenance and repair.
It will be further understood that pumping mechanisms in the past have been designed with regard to their mechanical function alone—that is to say, the process of their design has been entirely focused on providing a reliable method of raising and lowering a long pumping string within a shaft through which the liquid is itself raised on the upstroke of the pump. In the design of that mechanism no significant thought has been given to means of sensing the efficacy of the pumping operation or of reacting to special conditions that may strongly affect the loads on the pumping apparatus. For example, the mechanisms of the prior art do not generally incorporate within themselves the ability to sense and to react appropriately to conditions such as a dry well, a broken rod string or a stuck valve. Such conditions could only be discovered or diagnosed as a result of routine inspection and maintenance, and before that discovery the untreated condition will not only have stopped the pumping process, but may have been the cause of considerable damage to the pumping mechanism.
In recent years a variety of alternative systems have been devised, some using directly-applied hydraulic power to raise and lower the polished rod. Other alternatives have proposed the use of direct-acting linear electric motors, although none of them has been commercially successful. Co-pending patent applications GB 0713531.2 and PCT/GB2007/003482, the contents of which are incorporated herein by reference, consider that body of prior art and describe the general form of an electrical mechanism that is able both to fulfil the mechanical task of pumping and to deduce, in real time, the conditions occurring within the pump and its surroundings at the base of the well.
Other generally relevant prior art includes GB 0802964.7. This reference is specifically related to a pump or artificial lift mechanism and not for the design of an electrical machine of any particular kind. Likewise, WO 2008/032080 A2 is specifically directed to an electrical machine and not an artificial lift mechanism or pump of the type described in GB 0802964.7. WO 99/14724 although generally relevant, describes a pump or artificial lift mechanism. In U.S. Pat. No. 4,353,220, there is a description of a mechanism that deliberately makes the compressor a resonant machine along the operating axis of the machine. This is impossible for any artificial lift mechanism because the steel rod string between the electrical driver and the driven pump is usually about 6000 feet long and has a set of resonant frequencies that are naturally much greater than any operating frequency of the mechanism. Further, the rod string is surrounded by a viscous liquid that acts to damp out any such vibration. There is therefore no advantage in attempting to increase the efficiency of the artificial lift mechanism by inducing a longitudinal resonance in the rod. The gas spring in GB 0802964.7 stores and recycles energy by slow changes in the PV product which obtains when the machine is at mid-stroke. As a result, part of the braking force and part of the acceleration force needed at each end of the machine stroke can be taken from the energy differential in the compressed (or extended) spring. From an analysis of the prior art, it would appear that there is nowhere any reference to or illustration of, the specific use of a planar wireless motor.